The present invention relates to a test tube for the examination of samples in general, and more particularly to a test tube for the examination of urine samples for the amount of glucose present therein, particularly in clinical environment.
In this connection, it is already known to contact the sample with a first reagent that reacts with an ingredient the amount of which is to be established in a reaction in which hydrogen peroxide is formed, and with a second reagent which, in cooperation with the so formed hydrogen peroxide, causes an indicator dye to change its color for indicating the amount of the ingredient in the sample.
An example of such an examination is the examination of urine for the glucose contents thereof by means of a testing strip. In this case, the first reagent is glucose oxidase which oxidizes the glucose to .delta.-gluconolactone in the presence of oxygen (in the ambient air). During this reaction, hydrogen peroxide (H.sub.2 O.sub.2) is released. This reaction is as follows: ##STR1##
The hydrogen peroxide which is formed during this reaction (1) oxidizes, for instance, a colorless indicator dye which changes or develops its color during its oxidation, such as, for instance, a colorless chromogen, for instance, o-tolidine which, when oxidized, acquires a blue color. This reaction takes place in the presence of peroxidase as follows: ##STR2##
It is also known (compare, for instance, Kohl, Storungen der Glucose-Anzeige durch Ascorbinsaure bei Teststreifen, Laboratoriumsblatter 27 (1977), pp 103-108) that the presence of ascorbic acid in the urine sample leads to inaccurate or totally false negative results, inasmuch as the hydrogen peroxide which is released during the oxidation of glucose to .delta.-gluconolactone and the amount of which determines, after the reaction (2), the degree of the coloring of the testing strip, reacts with the ascorbic acid and oxidizes the same to dehydroascorbic acid, which latter is a colorless substance. This reaction takes place as follows: ##STR3##
This, consequently, means that the hydrogen peroxide which develops in the course of the reaction (1) is consumed by the reaction (3) with the ascorbic acid, so that the so consumed hydrogen peroxide is unavailable for causing a change in the color of the indicator dye and thus for the indication of the presence of glucose in the urine sample. This, of course, means that the presence and amount of glucose in the urine sample is not indicated at all, or is not indicated to a degree corresponding to the actual condition.
This problem has already been recognized before and, in order to eliminate this error during the determination of glucose content by means of a testing strip, it has been proposed (see, for instance, page 105 of the above publication) to additionally provide an ascorbic acid testing area in the testing strip, so that the evaluation of urine samples which contain ascorbic acid can be avoided. However, this method leads merely to the recognition of such urine samples which are unsuited for the performance of the rapid test using the testing strip, and cannot be used to determine the actual amount of glucose in such urine samples.